1. Field of the Invention
The present invention relates generally to methods and devices for the non-destructive evaluation of materials. The present invention relates more specifically to a magnetostrictive sensor based system for the long-range guided-wave inspection of longitudinal structures and a method for compensating for signal attenuation through intelligent variable time-gain control signal amplification.
2. Description of the Related Art
Introduction
The ongoing ability of structural components to function in their intended manner often depends upon the maintenance of their material integrity. Various techniques are used to investigate and monitor the integrity of longitudinal structural objects. Non-Destructive Evaluation (NDE) techniques are important tools to accomplish this investigation and monitoring. NDE techniques range from ultrasonic test systems and methods to electromagnetic (EM) test systems and methods. A highly beneficial feature for some NDE techniques is their ability to investigate and monitor a large (especially long) structure from a single point or a small number of points on the structure. Many such “large structures” are longitudinal in nature (pipes, cables, tubes, plates, and conduits for example). Such structures present specific problems for the investigation and/or monitoring of locations as much as 100 feet or more from the placement of a sensor in an NDE type system.
Long-Range Guided-Wave Inspection
Long-range guided-wave inspection of structures is a recently developed NDE inspection technology that can examine a long length (such as 100 feet) of a longitudinal structure (such as pipes, tubes, steel cables, and plates) quickly, and therefore economically, from a given sensor location. At present, there are two well established guided-wave inspection technologies. One is commonly referred to as magnetostrictive sensor (MsS) technology, and has been pioneered by Southwest Research Institute (SwRI) of San Antonio, Tex. (SwRI is the Assignee of a series of U.S. patents covering MsS based Long-Range Guided-Wave Inspection techniques, including U.S. Pat. Nos. 5,456,113, 5,457,994, 5,581,037, 5,767,766, 6,212,944, 6,294,912, 6,396,262, 6,429,650, 6,624,628, 6,917,196, and as well as additional patents pending).
An example of the functionality of MsS Systems, as described above, can be found in U.S. Pat. No. 6,917,196 issued to Kwun et al. on Jul. 12, 2005 entitled Method and Apparatus Generating and Detecting Torsional Wave Inspection of Pipes and Tubes. This patent describes one approach for implementing MsS Techniques for the NDE of pipes or tubes. In this case, a MsS generates guided-waves which travel in a direction parallel to the longitudinal axis of the pipe or tube. This is achieved (in this particular sensor system) by using a magnetized ferromagnetic strip pressed circumferentially against the pipe or tube. The guided-waves are generated in the strip, are coupled to the pipe or tube, and propagate along its length. Detected guided-waves are coupled back to the thin ferromagnetic strip and may include reflected waves representing defects in the pipe or tube. The full disclosure of U.S. Pat. No. 6,917,196 is incorporated herein by reference.
A second NDE technique used in conjunction with longitudinal structures is commonly referred to as Lamb wave inspection technology. Commercial systems implementing such techniques are marketed under the names Teletest® and Wavemaker®. These systems have been developed by the Imperial College of Science, Technology of Medicine of London, England. These techniques are typified by the system described in U.S. Pat. No. 6,148,672 entitled Inspection of Pipes issued to Crawley et al. on Nov. 21, 2000 and assigned to Imperial College of Science, Technology of Medicine.
The MsS based systems described above generate and detect guided-waves in ferromagnetic materials (such as carbon or alloyed steel) without requiring direct physical contact to the material. Lamb wave based systems on the other hand, generate and detect guided-waves by coupling the waves to an array of piezoelectric sensors in direct physical contact to the material. The MsS is applicable for inspection of various structures including pipes, tubes, plates, and steel cables, whereas the Lamb wave method is primarily used for inspection of pipe from the outside. Both technologies are now in commercial use.
There are a number of advantages to using the magnetostrictive effect in generating and detecting guided-waves for NDE applications. These advantages include; (a) the sensitivity of the magnetostrictive sensors, (b) the mobility of the magnetostrictive sensors, (c) the absence of a need to couple the sensor to the material being investigated, (d) the long-range of the mechanical waves in the material under investigation, (e) the ease of implementation, and (f) the low cost of implementation. The use of magnetostrictive sensors (MsS) in the NDE of materials has proven to be very effective in characterizing defects, inclusions, and corrosion within various types of ferromagnetic and non-ferromagnetic structures. Since guided-waves can propagate long distances (typically 100 feet or more) the magnetostrictive sensor technology can inspect the global volume of a structure very quickly. In comparison, other conventional NDE techniques, such as ultrasonics and EMAT current, inspect only the local area around the sensor.
In general, guided-wave inspection and monitoring techniques involve launching a pulse of guided-waves along the length of pipe and detecting signals reflected from defects such as corrosion damage or cracks back to the position from which the original wave was transmitted. A system for carrying out such an inspection or monitoring of a structure is shown in FIG. 1 and described in more detail below. From the occurrence time of the defect signal and the signal amplitude, the axial location and severity of the defect may be determined. The magnetostrictive sensor based guided-wave inspection technique can afford a 100% volumetric inspection of a long length of piping and has been used effectively on more than 500 feet of straight piping structure in one direction above ground with painting, from a single probe location. Such guided-wave inspection systems are therefore particularly useful for remote inspection of difficult-to-access areas (such as those under insulation or at high elevations) by launching and detecting waves from a remote but accessible location.
Return Signal Analysis—Compensating for Attenuation
Guided-wave inspection techniques can provide comprehensive information on piping conditions with minimal preparation and inspection time. These systems are therefore gaining rapid acceptance as cost-effective inspection methods in various industries including gas, oil, petrochemical, and electric power, where piping is a primary component of the equipment and facilities that serve to transport the product across great distances. One difficulty associated with the inspection or monitoring of longitudinal structures of significant length involves the attenuation of the amplitude of the propagated and reflected signals over distance and time. Because of this attenuation, the guided-waves propagating in a structure become progressively smaller in amplitude the further the waves propagate. If the attenuation is constant, the signal amplitude would decrease exponentially with time or, equivalently, with the traveling distance. Consequently, the guided-wave signals (or echoes) from similar reflectors in a structure (such as girth welds in piping) appear smaller as their distance from the guided-wave probe increases. To maintain the detection sensitivity of the guided-wave so as to be approximately the same over its testing range and, thus, to facilitate subsequent data analyses and interpretation, it is desirable to correct (or compensate for) the attenuation effects on the detected signals.
To achieve the attenuation correction mentioned above, ultrasonic testing systems use a method called time-gain control (TGC), also known as time-gain compensation, time-controlled gain, time-corrected gain, distance-gain control, etc. Since ultrasonic testing systems are used primarily for the short-range (a few feet), local examination of materials, TGC methods employed in these systems do not directly translate as suitable for long-range guided-wave inspection systems. TGC as used with short-range ultrasonic type systems is able to make certain assumptions about the character and structure of the material being inspected that cannot be made with the longer distances of irregular structures that are often inspected and monitored with guided-wave systems.
The purpose of the present invention therefore is to provide systems and methods for the application of time-gain control techniques to long-range guided-wave inspection and monitoring systems. It is therefore an object of the present invention to provide systems and methods that condition a sensor signal from a guided-wave inspection or monitoring system so as to improve the discrimination of defect signals and therefore the detection and location of defects in the structure under investigation.